(1) Field of the Invention
This invention relates to a laminated thermoplastic film. More particularly, it relates to a base film suitable for a super-high density magnetic recording medium having very good running properties.
(2) Description of the Prior Art
The increase in recording density in magnetic recording media has led to a demand for increased surface smoothness in the thermoplastic resin film used for the recording media. On the other hand, the need to improve operation adaptability at the film-forming step or the magnetic tape-preparing step has led to a demand for good slip characteristics for the surface of the film and has made the slip properties of the magnetic tape crucial. Thus, it is required that a thermoplastic resin base film suitable for a magnetic recording medium have excellent smoothness and good slip characteristics.
As means for satisfying these requirements, there has been adopted a method in which fine particles are incorporated in a starting polymeric material for formation of a smooth film. According to this conventional technique, however, improvement of either the smoothness or slip characteristics results in degradation of the other. It has heretofore been impossible to improve both properties simultaneously.
A conventional magnetic recording tape of the coating type, is prepared by coating a base film with a magnetic paint comprising a magnetic powder and an organic polymer binder. The coating layer has a relatively large thickness, i.e., ordinarily 3 to 6.mu.. Accordingly, convexities and concavities sufficient to cause drastic reduction of the electromagnetic performances seldom occur in the magnetic layer. Therefore, in a conventional base film for a magnetic recording medium, a method has been employed wherein finely divided particles are incorporated in the starting polymeric material or internal particles are formed in the starting polymeric material from the polymerization catalyst residue, whereby slip characteristics are imparted to the film.
However, this coating type magnetic recording medium does not meet recent requirements for superhigh magnetic recording density. Accordingly, a metallized film tape having a highly magnetic, thin metal surface layer formed by vacuum evaporation or sputtering without the use of a binder has become popular as a super-high recording density tape instead of the magnetic recording medium of the coating type.
The thickness of the magnetic metal layer of the above-mentioned metallized film tape is usually 0.05 to 0.2.mu., mucn thinner than that in the magnetic tape of the coating type. Therefore, the surface configuration of the base film used is completely reproduced on the surface of the metallized film tape. Accordingly, if the surface roughness of the base film is large, the electromagnetic performance of the metallized film video tape is extremely reduced. In fact, if a metallized film video tape formed from a finely divided particle-incorporated base film is used in a video tape recorder (VTR), no image is reproduced. Therefore, this tape cannot be used as a video tape.
In view of the electromagnetic performance, a base film having quite a smooth surface is preferred. Practically, however, a problem arises with such a smooth surface film. Specifically, a vacuum-deposited metal film formed on a base film having a smooth surface is inferior in surface slip characteristics. Therefore, unless, for example, a protecting film layer is formed thereon, many scratches are inevitably produced on the surface of the vacuum-deposited metal film by contact with guide pins, heads, and the like in the tape-forming process after the vacuum evaporation or during the use thereof in a VTR.
As a base film for the high density magnetic recording medium in which the foregoing defect is eliminated, reduction of the electromagnetic performance is minimized, and the slip characteristics are improved, we previously proposed, in U.S. Pat. No. 4,233,352, a polyester film having on at least one surface thereof a discontinuous film formed of worm-like nodules composed of a mixture of a water-soluble polymer and a polysiloxane or styrene-butadiene rubber. The above-mentioned problem has been substantially solved by this proposal and a good image can be reproduced even when a metallized film is used as a video tape if the metallized layer is formed on the surface on which the discontinuous film is shaped.
In U.S. Pat. No. 4,233,352, it is taught that the running property is improved by forming a coating layer on a smooth surface. However, it was found that, under severe running conditions, for example, when the frequency of repeated use is extremely increased, the running properties and durability are inferior to those of an ordinary fine particle-containing film. Accordingly, research was conducted with a view to eliminating this defect. We found that this defect can be eliminated in a film having a specific discontinuous film formed on one surface, on which the magnetic recording layer is to be formed, and having on the opposite surface a specific surface roughness due to fine particles having a regulated size.
In a base film wound in the form of a roll, the configuration of the coarse surface is transferred to the contacted smooth surface to degrade the smoothness of the smooth surface. When a vacuum-deposited tape is prepared by running the film on a smooth cooling can, the front smooth surface is influenced by the coarse surface, whereby the smoothness is degraded. Moreover, when the formed magnetic tape is wound in the form of a roll, the configuration of the coarse surface is transferred to the smooth surface, resulting in reduction of the smoothness.